1. Field of the Invention
The invention relates to identification of electro-fusion fittings.
Such fittings are well-known and such fittings of one kind are used for joining lengths of thermoplastics pipe e.g. polyethylene pipe end-to-end. There are also other kinds of electrofusion fittings e.g. saddle-T kind which can be joined to the side surface of a main pipe to provide a stub to which a branch pipe can subsequently be joined.
Such fittings have an electrically conductive heater element which is energised, in use, by means of a control unit to which the fitting is connected using a lead or leads each of which terminates in a connector which is engageable with a respective one of two heater terminals on the fitting. In order that human error be eliminated as far a possible it is required that the fitting be identified from the range of fittings each of which has a predetermined characteristic, such as the time of electrical energisation for example, by the control unit.
2. Description of the Related Art
Such fittings are described in European Patent Specification No. 0076043 (Vectstar Limited) which shows a resistor connected between a terminal of the heater element and an auxilary terminal. The fitting described has two identical resistors, one connected to each heater element terminal so that one or other of the two resistors is effectively connected to the circuitry of the control unit whichever way round the fitting is connected to the leads. In other words, the provision of two resistors means that the fitting is not "handed" and is symmetrical so far as the control unit is concerned. The two resistors are not used together to determine the voltage used to identify the fitting. Either one or the other is used alone to determine that voltage.
Before power is applied to the fitting, a connection is made by the control unit which makes a voltage divider network including the resistor and a fixed resistor. The network is energised by a reference voltage and a voltage across the fixed resistor is measured. This voltage is used to identify the characteristic of the fitting e.g. the time of electrical energisation of the heating element of the fitting.
By using a single resistor to determine the voltage used to identify the fitting it is not possible to identify a range of fittings, say thirty or so, without using carefully chosen resistances. Such resistances are not available in the course of manufacture of electro-fusion fittings. If ordinary resistances are used having the normal error in their value, the differences which must be chosen between different values of resistors, to allow for the errors, would be excessively great.
By using a single resistor to determine the voltage used to identify the fitting it is not possible to identify a range of fittings, say thirty or so, without using carefully chosen resistances. Such resistances are not available in the course of manufacture of electro-fusion fittings. If ordinary resistances are used having the normal error in their value, the differences which must be chosen between different values of resistors, to allow for the errors, would be excessively great.